If you take medicine as an example, all drug development comes with unpredictable side effects. The process for putting the safe label on a drug is through testing it on 10s of thousands of people – it's extremely primitive and only works if we're willing to accept catastrophic failures from time to time. Even when something goes wrong, we often don't know why. And even when we know why, Pharma companies have so much invested that they often hide it for profit reasons.
Or: When we started producing CFCs we had no idea that we'd weaken the ozone layer. How could we? It's a miracle that the causal link was found at all.
The problem of having only one planet makes climate engineering extremely risky. Obviously it depends on what method is used. Carbon capture is ~easy to test for side effects in isolation, whereas chemicals in the stratosphere or a space sunshade is harder.
Oh sorry, i forgot we can techno-fix that with iron-seeding and massive alcali dumps.
So to make it short: these "solutions" provide FF companies more arguments to pump more oil.
BTW we are already MASSIVELY reducing forcing (0.8W/m2) with sulfur aerosols from sea shipping. When that goes down expect even faster heating, with consequences beyond what the bleakest models predict.
I leave you the size of the sun shade to get that negative forcing as an exercise.